60 ORGANIC EVOLUTION 



It is not surprising, in view of these difficulties, that the 

 statistical records are very scant. 



There is no doubt of the reality and great importance of 

 sexual selection among mankind, and to the author its opera- 

 tion seems probable at least among birds, fishes, and spiders. 



SEGREGATION 



Natural selection and sexual selection, and also the 

 inheritance of parental modifications which we will discuss 

 later, are primary factors in evolution. Segregation, to 

 which we have already made some reference, is not a pri- 

 mary factor in the development of species, but, acting in 

 connection with the primary factors, it greatly modifies the 

 results produced by these. Anything which divides a 

 species into groups which do not freely interbreed is said to 

 segregate the members of the species into these subdivisions. 

 In connection with one of the objections urged against the 

 effectiveness of natural selection we spoke of some of the 

 things that may cause segregation within a species. It is 

 well to treat the subject a little more fully. 



Segregation may be due to any of a number of causes. 

 If only anything operates to prevent free interbreeding 

 between any of the individuals of a species, it is a true 

 cause of segregation. 



The cause of segregation may be geographical. A 

 species of wide distribution is likely to be divided into 

 groups, which do not habitually interbreed, by the inter- 

 vention of rivers, or mountain ranges, or deserts, or oceans 

 between the different groups. The foxes of Europe differ 

 from those of America, and probably this divergence from 



